The present invention relates to a loudspeaking telephone system for performing speeches by the use of a speaker and a microphone, and more particularly to a loudspeaking telephone system in which variable attenuations to be inserted into both the transmitting and receiving channels are determined through arithmetic operation by the use of digital signals resulting from analog to digital conversion (hereinafter simply referred to as A/D conversion) and controlled on the basis of results of the arithmetic operation.
As is well recognized in the art, a voice switch circuit (hereinafter, often abbreviated as VS circuit) used in the above loudspeaking telephone system has a function of preventing howling of a loudspeaking telephone circuit. A typical voice switch circuit comprises a comparator for comparing the levels of the voice signals in the transmitting circuit and the receiving circuit, a variable loss circuit for the transmitting circuit responsive to the output from the comparator for inserting a loss in the transmitting circuit and a variable loss circuit for the receiving circuit responsive to the output from the comparator for inserting a loss in the receiving circuit. Where a received signal is present, a portion of the received signal is broadcasted by a loudspeaker and enters into a microphone as a sound signal, but under these conditions the comparator judges that the signal level in the receiving circuit is higher than the signal level in the transmitting circuit whereby a definite loss is inserted in the transmitting line variable loss circuit so as to decrease to zero the loss in the receiving line variable loss circuit. As a consequence, it is possible to hear the received signal at an adequate level without it being attenuated by the receiving line variable loss circuit. At the same time, the transmitting circuit variable loss circuit inserts a loss in the transmitting circuit thereby preventing howling. The condition in which a loss is inserted in the transmitting line as above described is herein termed the "receiving state". On the other hand, when there is no received signal and only the transmitting signal is supplied to the microphone the comparator functions to insert a loss in the receiving circuit variable loss circuit and to reduce to zero the loss in the transmitting circuit variable loss circuit for preventing the howling. The condition in which a loss is inserted in the receiving circuit is herein termed the "transmitting state". Where a speech is transmitted through the microphone under said receiving state, the level of the sum of a portion of the transmitted signal and a portion of the received signal which is supplied to the microphone from the loudspeaker as the round signal becomes higher than the level of a signal applied to the comparator from the receiving circuit with the result that the comparator changes the condition of the loudspeaking telephone circuit to the transmitting state thus enabling offering of a transmitting signal.
In the VS circuit described above, during the receiving state, as the signal level in the receiving circuit which is applied to the comparator is substantially higher than the signal level in the transmitting circuit, even when a speech is offered to the microphone during receiving, it would be extremely difficult to change the VS circuit from the receiving state to the transmitting state. Even when the transmitting state is attained, the initial syllable or the speech burst of the transmitting signal is substantially cut off by the transmitting variable loss circuit thus greatly distorting the speech. Such problems regarding difficulty in the offering and cut off of the initial syllable of the speech burst are also present in a case where the offering of a received signal is made during the transmitting state. In this manner, the prior art voice switch circuit has a serious defect that the initial syllable of the speech burst is lost due to the difficulty in the offering of the transmitted and received signals.
As one approach to these problems, U.S. Pat. No. 3,952,166 discloses a loudspeaking telephone circuit comprising a first rectifier-filter circuit for rectifying and filtering a transmitted signal through a transmittng circuit, a second rectifier-filter circuit for rectifying and filtering a received signal by a receiving circuit, a main comparator for comparing the outputs from the first and second rectifier-filter circuits, a resistance attenuator for attenuating at least one of the outputs from the first and second rectifier-filter circuits, a control comparator for comparing the output from the resistance attenuator with the unattenuated outputs from the first and second rectifier-filter circuits, a memory device for memorizing the output from the control comparator, and transmitting circuit variable loss means and receiving circuit variable loss means which provide an insertion loss to a speech circuit by utilizing the output from the memory device and the main comparator as a parameter.
However, the conventional system of U.S. Pat. No. 3,952,166 still has such a defect as will be described in the following.
First of all, specifically, when the distance between the mike and the speaker is shortened during the speech using the speaker and the mike so that the acoustic coupling condition is deteriorated worse than before, the previous control remains stored in the memory circuit so that the stable receiving state cannot be maintained and what appears to be a malfunction (which will be called the "receiving blocking") takes place and a howling is caused in an extreme case. Secondly, when the side tone coupling condition in the course of connecting the circuit system with the remote subscriber is deteriorated, the voice switching circuit cannot maintain the stable "transmission state" similarly to the aforementioned first defect and what appears to be a malfunction (which will be called the "transmission blocking") takes place and a howling is caused in an extreme case. Thirdly, by the influences of the dispersion in the variable resistance characteristics belonging to the field effect transistors of each variable loss circuit, there are caused in the course of the switching between the "transmitting state" and the "receiving state" of the voice switching circuit a howling and the voice is cut more than necessary. Fourthly, the changing characteristics of the variable loss circuit during the switching between the "transmitting state" and the "receiving state" of the voice switching circuit depend upon the variable resistance characteristics belonging to the field effect transistors thereby to make it difficult to realize desired characteristics. Fifthly, the number of the parts of the circuit is increased, and memory capacitors are required to invite a disadvantage in size reduction and in economy.